Semiconductor wafer processing method and semiconductor wafers produced by the same

ABSTRACT

A method of processing a semiconductor wafer sliced from a monocrystalline ingot comprises at least the steps of chamfering, lapping, etching, mirror-polishing, and cleaning. In the etching step, alkali etching is first performed and then acid etching, preferably reaction-controlled acid etching, is performed. The etching amount of the alkali etching is greater than the etching amount of the acid etching. Alternatively, in the etching step, reaction-controlled acid etching is first performed and then diffusion-controlled acid etching is performed. The etching amount of the reaction-controlled acid etching is greater than the etching amount of the diffusion-controlled acid etching. The method can remove a mechanically formed damage layer, improve surface roughness, and efficiently decrease the depth of locally formed deep pits, while the flatness of the wafer attained through lapping is maintained, in order to produce a chemically etched wafer having a smooth and flat etched surface that hardly causes generation particles and contamination.

This is a divisional of application Ser. No. 09/207,193 filed Dec. 8,1998, U.S. Pat. No. 6,239,039, which application is hereby incorporatedby reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an improvement on the method ofremoving, through chemical etching, a damaged layer that is generated onthe surface of a monocrystalline silicon wafer during a process ofproducing the wafer.

2. Description of the Related Art

A conventional process of producing a mirror-polished semiconductorwafer typically comprises the steps of slicing a monocrystalline ingotof silicon or the like to obtain a semiconductor wafer; and chamfering,lapping, acid etching, mirror-polishing, and cleaning the slicedsemiconductor wafer. Depending on the required specifications, thesequence of steps is changed; some steps are repeated a plurality oftimes; or other steps such as heat treatment and grinding are added toor replace the above-described steps. Thus, a variety of kinds of stepsare performed in accordance with the specifications.

Among the above-described steps, acid etching is performed for thepurpose of removing a surface damaged layer introduced in the course ofmechanical machining steps such as slicing, chamfering, and lapping. Inthe acid etching step, the surface of a wafer-is etched to a depth of afew to a few tens of microns through use of mixed acid aqueous solutioncomposed of hydrofluoric acid, nitric acid, acetic acid, and water.However, acid etching involves the following problems:

1) The flatness of a wafer after lapping—which is indicated by thicknessvariation represented by, for example, TTV (Total Thickness Variation)(μm) or LTV_(max) (Local Thickness Variation) (μm)—deteriorates as theetching amount increases.

2) A waviness of a millimeter order or an uneven region called “peel” isgenerated on an etched surface.

3) Harmful NO_(x) is generated due to etching.

In consideration of these problems, alkali etching is used in somecases.

Alkali etching has the following advantages:

a) Flatness established by lapping is maintained after etching.

b) Generation of harmful gas is suppressed.

However, alkali etching has the following disadvantages:

i) If foreign mater enters pits locally existing on an etched surfaceand having a depth of a few microns and a diameter of a few to twelve orthirteen microns, the foreign matter causes generation of particlesand/or contamination in a subsequent step.

ii) Since deep pits exist and surface roughness (Ra) increases, apolishing stock removal in a subsequent step of mirror-polishing(mechano-chemical polishing) must be increased.

iii) Since an etched surface has a sharp uneven shape compared to asurface etched through acid etching, the unevenness itself serves as asource of particles.

Accordingly, particles generated in a subsequent step and a polishingstock removal in a mirror-polishing step can be decreased if etchingtreatment can be performed while flatness attained through lapping ismaintained, so as to remove a mechanically formed damage layer, improvethe surface roughness, efficiently decrease the depth of deep pitslocally formed due to the etching, and smooth the uneven shape of thesurface.

SUMMARY OF THE INVENTION

The present invention has been accomplished to solve the above-mentionedproblems, and an object of the invention is to provide a method ofprocessing a semiconductor wafer which can remove a mechanically formeddamage layer, improve surface roughness, and efficiently decrease thedepth of locally formed deep pits, while the flatness of the waferattained through lapping is maintained, in order to produce a chemicallyetched wafer (CW) having a smooth and flat etched surface that hardlycauses generation of particles and contamination.

Another object of the invention is to provide a semiconductor waferprocessed through the above-described processing method.

To achieve the above object, the present invention provides a method ofprocessing a semiconductor wafer sliced from a monocrystalline ingot.The method comprises at least the steps of chamfering, lapping, etching,mirror-polishing, and cleaning and is characterized in that in theetching step alkali etching is first performed and then acid etching isperformed, and that an etching amount of the alkali etching is greaterthan an etching amount of the acid etching.

In the etching step of the processing method of the present invention,after the step of lapping alkali etching is first performed in order toremove a mechanically formed damage layer, while the flatness of thewafer attained through lapping is maintained, and subsequently, acidetching is performed in order to decrease the depth of locally formeddeep pits remaining after the alkali etching and to improve the surfaceroughness and the sharp uneven shape.

At this time, the etching amount of the alkali etching must be setgreater than the etching amount of the acid etching because of thefollowing reasons. That is, in order to decrease the depth of locallyformed deep pits remaining after the alkali etching, the etching amountof the alkali etching must be increased to a certain level, which isgreater than the etching amount of the acid etching required fordecreasing the rate of generation of faults such as stain stemming fromunevenness in etching and for improving flatness.

Preferably, before being subjected to the acid etching a wafer that hasundergone the alkali etching is immersed into aqueous solution ofhydrogen peroxide.

The surface of a wafer that has undergone the alkali etching is activeand hydrophobic, so that foreign matter easily adheres and dirties thewafer. However, if the surface of the wafer is oxidized throughimmersion into aqueous solution of hydrogen peroxide and thus madehydrophilic, particles hardly adhere to the wafer surface.

Preferably, the etching amount of the alkali etching is 10-30 μm, andthe etching amount of the acid etching is 5-20 μm.

In the alkali etching, there is a tendency that the depth of locallyformed deep pits remaining after the alkali etching decreases withincreasing etching amount, and that the surface roughness becomes higherwith increasing etching amount of the alkali etching. Therefore, theetching amount of the alkali etching is maintained within theabove-described range. In the acid etching, as the etching amountincreases, the stain generation rate decreases considerably although theflatness deteriorates. Therefore, the etching amount of the acid etchingis maintained within the above-described range.

Preferably, the etchant used in the alkali etching is an aqueoussolution of NaOH or KOH, and the etchant used in the acid etching is amixed acid aqueous solution composed of hydrofluoric acid, nitric acid,acetic acid, and water.

When such etchants are used, etching is performed effectively andreliably in both the alkali etching and the acid etching, and therespective etching amounts can be controlled with relative ease. Inaddition, the etching can be performed at low cost.

In the present specification, each specific value used in relation toetching amount represents the sum of the thicknesses of layers removed,through etching, from opposites surfaces of a wafer.

Preferably, the acid etching is reaction-controlled acid etching.

When the acid etching is of a reaction-controlled type, the flatness canbe further improved through suppression of waviness, while realizing adecrease in the depth of deep pits locally remaining after the alkalietching and improvement of the surface roughness and the sharp unevenshape.

Preferably, in the reaction-controlled acid etching, there is used anetchant obtained through addition of 20-30 g/l of silicon into a mixedacid aqueous solution composed of hydrofluoric acid, nitric acid, aceticacid, and water.

When such etchant is used, etching is performed effectively andreliably, and the etching amount can be controlled with relative ease.In addition, the etching can be performed at low cost.

The present invention provides another method of processing asemiconductor wafer sliced from a monocrystalline ingot. The methodcomprises at least the steps of chamfering, lapping, etching,mirror-polishing, and cleaning and is characterized in that in theetching step reaction-controlled acid etching is first performed andthen diffusion-controlled acid etching is performed, and that an etchingamount of the reaction-controlled acid etching is greater than anetching amount of the diffusion-controlled acid etching.

In the etching step of the processing method of the present invention,reaction-controlled acid etching is first performed for a lapped waferin order to remove a mechanically formed damage layer, while theflatness of the wafer attained through lapping is maintained, andsubsequently, diffusion-controlled acid etching is performed in order todecrease the depth of deep pits remaining after the reaction-controlledacid etching and to improve the surface roughness and the sharp unevenshape.

At this time, the etching amount of the reaction-controlled acid etchingmust be set greater than the etching amount of the diffusion-controlledacid etching because of the following reasons. That is, in order todecrease the depth of locally formed deep pits remaining after thereaction-controlled acid etching, the etching amount of thereaction-controlled acid etching must be increased to a certain level,which is greater than the etching amount of the diffusion-controlledacid etching required for decreasing the rate of generation of faultssuch as stain stemming from unevenness in etching and for improvingflatness.

Preferably, the etching amount of the reaction-controlled acid etchingis 10-30 μm, and the etching amount of the diffusion-controlled acidetching is 5-20 μm.

In the reaction-controlled acid etching, there is a tendency that thedepth of locally formed deep pits remaining after the etching decreaseswith increasing etching amount, and that the surface roughness increaseswith increasing etching amount of the reaction-controlled acid etching.Therefore, the etching amount of the reaction-controlled acid etching ismaintained within the above-described range. In the diffusion-controlledacid etching, as the etching amount increases, the stain generation ratedecreases considerably although flatness deteriorates. Therefore, theetching amount of the diffusion-controlled acid etching is maintainedwithin the above-described range.

Preferably, in each of the reaction-controlled acid etching and thediffusion-controlled acid etching, there is used an etchant obtainedthrough addition of silicon to a mixed acid aqueous solution composed ofhydrofluoric acid, nitric acid, acetic acid, and water, and the siliconconcentration of the etchant used in the reaction-controlled acidetching is higher than that of the etchant used in thediffusion-controlled acid etching.

When such etchants are used, etching is performed effectively andreliably in both the reaction-controlled acid etching and thediffusion-controlled acid etching, and the respective etching amountscan be controlled with relative ease. In addition, the etching can beperformed at low cost.

Preferably, the silicon concentration of the etchant used in thereaction-controlled acid etching is 20-30 g/L, and the siliconconcentration of the etchant used in the diffusion-controlled acidetching is 5-15 g/l.

When the silicon concentration of the etchant used in thereaction-controlled acid etching is less than 20 g/l, the etchantbecomes a diffusion-controlled-type acid, so that flatness deteriorates.When the silicon concentration of the etchant used in thereaction-controlled acid etching exceeds 30 g/l, the etching ratedecreases, and a longer period of time is required for dissolvingsilicon in a mixed acid aqueous solution in order to prepare theetchant. Therefore, the silicon concentration of the etchant used in thereaction-controlled acid etching is preferably adjusted to fall withinthe range of 20 to 30 g/l. When such reaction-controlled type acidetchants are used, etching is performed effectively and reliably, andthe etching amount can be controlled with relative ease. In addition,the etching can be performed at low cost.

In the diffusion-controlled acid as well, small amount of silicon ispreferably dissolved into the mixed acid aqueous solution in order toprevent variations in the etching rate, which would otherwise be causedby variations in the composition of the solution. When the siliconconcentration is less than 5 g/l, a variation in the composition of thesolution causes a large variation in the etching rate. When the siliconconcentration is exceeds 15 g/l, the etching rate decreases, and thestate of the surface of a wafer after etching becomes similar to thatobtained through etching by use of a reaction-controlled-type acid, sothat the surface roughness increases. Therefore, the siliconconcentration of the etchant used in the diffusion-controlled acidetching is preferably adjusted to fall within the range of 5 to 15 g/l.

The present invention further provides a semiconductor wafer processedby either one of the above-described methods of the present invention.As described above, in one method of the present invention, alkalietching is first performed in order to remove a mechanically formeddamage layer, while the flatness of the wafer attained through lappingis maintained, and subsequently, acid etching is performed. Therefore,there can be obtained a semiconductor wafer in which the depth of deeppits remaining after the alkali etching is decreased and the surfaceroughness and the sharp uneven shape are improved. Especially, whenreaction-controlled acid etching is employed as the acid etching, thedegree of waviness decreases, so that a semiconductor wafer having aflatter surface can be produced.

The above-described wafer can be obtained through the other method ofthe present invention, in which reaction-controlled acid etching isfirst performed and then diffusion-controlled acid etching is performedand in which the etching amount of the reaction-controlled acid etchingis greater than the etching amount of the diffusion-controlled acidetching.

The present invention further provides a semiconductor wafer in which anLTV_(max) measured in cells of 20×20 mm is 0.3 μm or less, and themaximal value of pit depth is 6 μm or less. In this case, the averagevalue of waviness of the semiconductor wafer is preferably 0.04 μm orless.

As described above, according to the present invention, the flatness ofthe wafer attained through lapping is maintained; the degree of wavinessof the wafer surface after etching is decreased; deep pits are preventedfrom being locally generated; and degradation of surface roughness issuppressed. Thus, there is obtained a chemically etched wafer having asmooth and flat etched surface that hardly causes generation ofparticles and contamination such as stain. Therefore, the amount ofstock removal in a mirror-polishing step can be decreased, and theflatness of the wafer can be improved.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the relationship between etching amount anddepth of locally formed deep pits in wafers that have undergone alkalietching after lapping;

FIG. 2 is a graph showing the relationship between etching amount andTTV (flatness) in wafers that have undergone alkali etching afterlapping;

FIG. 3 is a graph showing the relationship between etching amount andsurface roughness (Ra) in wafers that have undergone alkali etchingafter lapping;

FIG. 4 is a graph showing the relationship between etching amount andTTV (flatness) in wafers that have undergone acid etching after lapping;

FIG. 5 is a graph showing the relationship between etching amount andstain generation rate in wafers that have undergone acid etching afterlapping;

FIG. 6 is an explanatory diagram showing a definition of waviness of thesurface of a wafer;

FIG. 7 is a graph showing the relationship between etching amount anddepth of locally formed deep pits in wafers that have undergonereaction-controlled acid etching after lapping;

FIG. 8 is a graph showing the relationship between etching amount andTTV (flatness) in wafers that have undergone reaction-controlled acidetching after lapping;

FIG. 9 is a graph showing the relationship between etching amount andsurface roughness (Ra) in wafers that have undergone reaction-controlledacid etching after lapping;

FIG. 10 is a graph showing the relationship between etching amount andLTV_(max) (flatness) in wafers that have undergone reaction-controlledacid etching after lapping.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention will now be described withreference to the drawings; however, the embodiments should not beconstrued as limiting the present invention.

The inventors of the present invention performed various studies on amethod of processing a semiconductor wafer, especially on the etchingmethod, that can produce a chemically etched wafer which maintains itsflatness attained through lapping and which has an etched surface thathardly causes generation of particles and contamination. Based on thestudies, the inventors of the present invention conceived a waferprocessing method in which alkali etching is first performed in order toremove a damage layer, while the flatness of the wafer attained throughlapping is maintained, and subsequently, acid etching is performed inorder to decrease the depth of remaining deep pits and to improve thesurface roughness while decreasing the degree of waviness, as well as awafer processing method in which reaction-controlled acid etching isperformed as the above-described acid etching. The present invention wasachieved on the basis of this concept and through thoroughinvestigations of other conditions.

First, alkali etching will be described in detail.

FIG. 1 shows the relationship between etching amount and depth oflocally formed deep pits in 8-inch wafers which were alkali-etched at85° C. through use of an aqueous 50% NaOH solution after being lapped byuse of #1200 lapping abrasive grains. FIG. 2 shows the relationshipbetween etching amount and TTV (flatness) in the 8-inch wafers, and FIG.3 shows the relationship between etching amount and surface roughness(Ra) in the 8-inch wafers.

The locally formed deep pits are formed as follows. When lappingabrasive grains stick in the surface of a wafer during lapping, thereare formed pits, whose diameter and depth are then increased due toalkali etching. Thus, deep pits are formed. When the concentration ofthe alkaline compound is low, the pit depth tends to increase. When theconcentration of an alkaline compound is high, the pit depth can bedecreased. However, in this case, the etching amount must be increased,resulting in a lowered efficiency. The pit depth is determined based onthe focal depth of an optical microscope. Such pits must be removedthrough polishing in a subsequent mirror-polishing step, and thereforethe polishing stock removal in the mirror-polishing step must be setgreater than the maximum value of the depth of such deep pits.Accordingly, the depth of pits is desirably decreased to a possibleextent.

TTV (Total Thickness Variation) (μm) represents the difference betweenthe thickness of a thickest portion and that of a thinnest portion of asingle wafer and is an indication of wafer flatness. LTV (LocalThickness Variation) (μm) represents the difference between thethickness of a thickest portion and that of a thinnest portion withineach of cells (typically 20×20 mm, or 25×25 mm) defined on a singlewafer. The LTV of each cell is called LTV_(cbc), and the maximum LTVwithin a single wafer is called LTV_(max), and these also serve asindications of wafer flatness.

Ra (μm) represents an arithmetical mean deviation of profile, which isthe most commonly used parameter of surface roughness.

As is understood from FIG. 1, in order to decrease the depth of locallyformed deep pits, the etching amount of alkali etching must be set tonot less than 10 μm. As is understood from FIG. 2, in order to decreaseTTV to 1 μm or less, the etching amount of alkali etching must be set to30 μm or less. As is understood from FIG. 3, in order to decrease Ra to0.25 μm or less, the etching amount of alkali etching must be set to 30μm or less. In consideration of the above, a suitable range of theetching amount of the alkali etching is 10 to 30 μm. Especially, anetching amount of about 20 μm is preferred because the depth of locallyformed deep pits approaches its minimal value (approximately 5 μm), andTTV and Ra do not increase greatly.

Next, the etching amount of acid etching was studied.

FIG. 4 shows the relationship between average etching amount and TTVafter etching in 8-inch wafers which were etched through use of a mixedacid (50% hydrofluoric acid: 70% nitric acid: 99% acetic acid=1:2:1(volume ratio)) after being lapped by use of #1200 lapping abrasivegrains. In this case, silicon is dissolved in the mixed acid etchantwith a concentration of 10 g/l to improve controlability of etchingrate. Consequently this acid etching is diffusion-controlled type.

FIG. 5 shows the relationship between etching amount and staingeneration rate due to uneven etching in wafers that were chemicallyetched through acid etching. The generation of stain was determinedthrough visual inspection under collimated light.

As is understood from FIG. 5, in order to avoid generation of stain, theetching amount of acid etching must be set to not less than 5 μm, and inorder to reliably eliminate stain, the etching amount of acid etchingmust be set to not less than 10 μm. As is understood from FIG. 4, inorder to decrease TTV to 1 μm or less, the etching amount of acidetching must be set to 20 μm or less. In consideration of the above, asuitable range of the etching amount of the acid etching is 5 to 20 μm,and the etching amount is preferably set to approximately 10 μm.

In the above description, the relationship between the etching amount ofalkali etching and its etching effect is discussed separately from therelationship between the etching amount of acid etching and its etchingeffect. However, in the present invention, alkali etching and acidetching are both used, and the acid etching is performed after thealkali etching in order to fully utilize the characteristics of bothetchings, so that a sufficient degree of etching effect is attained.

That is, alkali etching is first performed in order to remove amechanically formed- damage layer, while the flatness of the waferattained through lapping is maintained, and acid etching is thenperformed. Thus, the depth of locally formed deep pits remaining afterthe alkali etching can be decreased; the surface uneven shape can besmoothed in order to improve the surface roughness; and the staingeneration rate can be decreased.

At this time, the etching amount of the alkali etching must be setgreater than the etching amount of the acid etching because of thefollowing reasons. That is, in order to decrease the depth of locallyformed deep pits remaining after the alkali etching, the etching amountof the alkali etching must be increased to a certain level, which isgreater than the etching amount of the acid etching required fordecreasing the rate of generation of stain and for improving flatness.

In the present invention, a wafer that has undergone the alkali etchingis preferably immersed into aqueous solution of hydrogen peroxide beforebeing subjected to the acid etching. The surface of a wafer that hasundergone the alkali etching is active and hydrophobic, so that foreignmatter easily adheres and dirties the wafer. However, if throughimmersion into aqueous solution of hydrogen peroxide the surface of thewafer is oxidized and thus made hydrophilic, so that particles hardlyadhere to the wafer surface, which particles would otherwise contaminatean acid etchant used in the subsequent step.

The concentration of hydrogen peroxide is preferably set to 0.1-30%.Concentrations less than 0.1% cannot make the surface of a waferhydrophilic to a sufficient degree. A concentration as high as 30%provides a sufficient effect, and therefore concentrations higher than30% are disadvantageous from a viewpoint of economy.

Next, reaction-controlled acid etching will be described.

In the reaction-controlled acid etching, there is used an etchantobtained through dissolution of 20-30 g/l of silicon into a mixed acidaqueous solution composed of hydrofluoric acid, nitric acid, aceticacid, and water. The mixed acid aqueous solution causes an etchingaction relatively close to that of alkali etchant.

The etchant used in the reaction-controlled acid etching is called areaction-controlled acid etchant because its reaction speed iscontrolled or determined, in contrast to an ordinarily mixed acidaqueous solution used in ordinary acid etching, which is adiffusion-controlled acid.

When the reaction-controlled acid etching is combined with the alkalietching, alkali etching is first performed in order to remove amechanically formed damage layer, while the flatness of the waferattained through lapping is maintained, and the reaction-controlled acidetching is then performed. Thus, the depth of locally formed deep pitsremaining after the alkali etching can be decreased; the surface unevenshape can be smoothed in order to improve the surface roughness; and thestain generation rate can be decreased. Further, since the degree ofwaviness can be decreased compared to the case wherediffusion-controlled acid etching is performed, the flatness of thewafer can be further improved.

The above-described two-stage chemical etching; i.e., alkali etchingplus acid etching, according to the present invention enables easy andstable production of a semiconductor wafer which has a flatness(LTV_(max) in 20×20 mm cells) of 0.3 μm or less and a maximum pit depthof 6 μm or less.

Further, a semiconductor wafer can be processed to have an excellentflatness in a large area such that the average value of waviness is 0.04μm or less.

Next, another embodiment of the present invention will be described withreference to the drawings; however, the embodiment should not beconstrued as limiting the present invention.

The inventors of the present invention performed various studies on amethod of processing a semiconductor wafer that can produce a chemicallyetched wafer which maintains its flatness attained through lapping andwhich has an etched surface that hardly causes generation of particlesand contamination, especially studies on the etching method. Based onthe studies, the inventors of the present invention conceived a waferprocessing method in which reaction-controlled acid etching is firstperformed in order to remove a damage layer, while the flatness of thewafer attained through lapping is maintained, and subsequently,diffusion-controlled acid etching is performed in order to decrease thedepth of remaining deep pits and to improve the surface roughness. Thepresent invention was achieved on the basis of this concept and throughthorough investigations of other conditions.

In the reaction-controlled acid etching, there is used an etchantobtained through dissolution of 20-30 g/l of silicon into a mixed acidaqueous solution (for example, 50% hydrofluoric acid: 70% nitric acid:99% acetic acid=1:2:1). This etchant was found while the presentinventors studied the etching action of the above-described mixed acidaqueous solution, and causes an etching action relatively close to thatof alkali etchant. The present inventors decided to call the etchant a“reaction-controlled type acid” because the etchant dominantly effectsreaction-controlled acid etching, in contrast to the mixed acid aqueoussolution used in ordinary acid etching, which is a diffusion-controlledacid. The present inventors decided to call the ordinary mixed acidaqueous solution a “diffusion-controlled type acid.”

The etching through use of the reaction-controlled type acid has thefollowing advantages:

a) Flatness established by lapping is maintained after etching.

b) Etching through use of the reaction-controlled type acid can beperformed at temperatures near room temperature, whereas alkali etchingis typically performed at temperatures near 80° C.

However, the etching through use of the reaction-controlled type acidhas the following disadvantages:

i) If foreign mater enters pits locally existing on an etched surfaceand having a depth of a few microns and a diameter of a few to twelve orthirteen microns, the foreign matter causes generation of particlesand/or contamination in a subsequent step.

ii) Since deep pits exist and surface roughness (Ra) increases, apolishing stock removal in a subsequent step of mirror-polishing(mechano-chemical polishing) must be increased.

iii) Since an etched surface has a sharp uneven shape compared to asurface etched through diffusion-controlled acid etching (acid etchingthrough use of an ordinary mixed acid), the unevenness itself serves asa source of particles.

When the solution temperature during etching is lower than 20° C., theetching rate decreases, and when the solution temperature during etchingexceeds 45° C., diffusion-controlled acid etching occurs dominantly, sothat the flatness of an etched wafer deteriorates. Therefore, theetching temperature is preferably set to fall within the range of 20 to45° C.

When the silicon concentration of the etchant used in thereaction-controlled acid etching is less than 20 g/l,diffusion-controlled acid etching occurs dominantly, so that theflatness of an etched wafer deteriorates. When the silicon concentrationof the etchant used in the reaction-controlled acid etching exceeds 30g/l, the etching rate decreases, and a longer period of time is requiredfor dissolving silicon in a mixed acid aqueous solution in order toprepare the etchant. Therefore, the silicon concentration of the etchantused in the reaction-controlled acid etching is preferably adjusted tofall within the range of 20 to 30 g/l.

FIG. 7 shows the relationship between etching amount and depth oflocally formed deep pits in 8-inch wafers which were etched at 35° C.through use of a reaction-controlled type acid etchant obtained throughdissolution of 26 g/l of silicon into a mixed acid (50% hydrofluoricacid: 70% nitric acid 99% acetic acid=1:2:1 (volume ratio)) after beinglapped by use of #1200 lapping abrasive grains. FIG. 8 shows therelationship between etching amount and TTV in the 8-inch wafers. FIG. 9shows the relationship between etching amount and surface roughness (Ra)in the 8-inch wafers. FIG. 10 shows the relationship between etchingamount and LTV_(max) in the 8-inch wafers.

The locally formed deep pits are formed as follows. When lappingabrasive grains stick in the surface of a wafer during lapping, thereare formed pits, whose diameter and depth are then increased due toreaction-controlled acid etching. Thus, deep pits are formed. When theconcentration of silicon is low, the pit depth tends to increase. Whenthe concentration of silicon is high, the pit depth can be decreased.However, in this case, the etching amount must be increased, resultingin a lowered efficiency. The pit depth is determined based on the focaldepth of an optical microscope. Such pits must be removed throughpolishing in a subsequent mirror-polishing step, and therefore thepolishing amount in the mirror-polishing step must be set greater thanthe maximum value of the depth of such deep pits. Accordingly, the depthof pits is desirably decreased to a possible extent.

As is understood from FIG. 7, in order to decrease the depth of locallyformed deep pits, the etching amount of reaction-controlled acid etchingmust be set to not less than 10 μm. As is understood from FIGS. 8, 9,and 10, the etching amount of reaction-controlled acid etching must beset to 30 μm or less in order to decrease TTV to 1 μm or less, Ra to0.30 μm or less, and LTV_(max) to 0.50 μm or less. In consideration ofthe above, a suitable range of the etching amount of thereaction-controlled acid etching is 10 to 30 μm. Especially, an etchingamount of about 20 μm is preferred because the depth of locally formeddeep pits approaches its minimal value (approximately 10 μm), and TTVand Ra do not increase greatly.

Next, the etching amount of diffusion-controlled acid etching wasstudied.

In the diffusion-controlled type acid as well, a small amount of siliconis preferably dissolved into the mixed acid aqueous solution in orderto-prevent variations in the etching rate, which would otherwise becaused by variations in the composition of the solution. When thesilicon concentration is less than 5 g/l, a variation in the compositionof the solution causes a large variation in the etching rate. When thesilicon concentration exceeds 15 g/l, the etching rate decreases, andthe state of the surface of a wafer after etching becomes similar tothat obtained through etching by use of a reaction-controlled type acid,so that the surface roughness increases. Therefore, the siliconconcentration of the etchant used in the diffusion-controlled acidetching is preferably adjusted to fall within the range of 5 to 15 g/l.

As is understood from FIG. 5, in order to avoid generation of stain, theetching amount of the diffusion-controlled acid etching must be set tonot less than 5 μm, and in order to reliably eliminate stain, theetching amount of the diffusion-controlled acid etching must be set tonot less than 10 μm. As is understood from FIG. 4, in order to decreaseTTV to 1 μm or less, the etching amount of acid etching must be set to20 μm or less. In consideration of the above, a suitable range of theetching amount of the diffusion-controlled acid etching is 5 to 20 μm,and the etching amount is preferably set to approximately 10 μm.

In the above description, the relationship between the etching amount ofreaction-controlled acid etching and its etching effect is discussedseparately from the relationship between the etching amount ofdiffusion-controlled acid etching and its etching effect. However, inthe present embodiment, reaction-controlled acid etching anddiffusion-controlled acid etching are both used, and thediffusion-controlled acid etching is performed after thereaction-controlled acid etching in order to fully utilize thecharacteristics of both etchings, so that a sufficient degree of etchingeffect is attained.

That is, reaction-controlled acid etching is first performed in order toremove a mechanically formed damage layer, while the flatness of thewafer attained through lapping is maintained, and diffusion-controlledacid etching is then performed. Thus, the depth of locally formed deeppits remaining after the reaction-controlled acid etching can bedecreased; the surface uneven shape can be smoothed in order to improvethe surface roughness; and the stain generation rate can be decreased.

At this time, the etching amount of the reaction-controlled acid etchingmust be set greater than the etching amount of the diffusion-controlledacid etching because of the following reasons. That is, in order todecrease the depth of locally formed deep pits remaining after thereaction-controlled acid etching, the etching amount of thereaction-controlled acid etching must be increased to a certain level,which is greater than the etching amount of the diffusion-controlledacid etching required for decreasing the stain generation rate and forimproving flatness.

EXAMPLES

The present invention will be described by way of examples; however, thepresent invention is not limited thereto.

Example 1

The following etching treatment was performed for wafers having adiameter of 8 inches that had undergone lapping (#1200 lapping abrasivegrains).

First, the wafers were immersed in an NaOH aqueous solution(concentration: 50 wt. %) at 85° C. for 450 seconds in order to performalkali etching with a target etching amount being set to 20 μm.Subsequently, the wafers were dipped into an aqueous solution of 0.3%hydrogen peroxide in order to make the surface of the wafershydrophilic. Finally, the wafers were immersed into a mixed acid (50%hydrofluoric acid: 70% nitric acid: 99% acetic acid=1:2:1 (volumeratio)) in order to perform acid etching with a target etching amountbeing set to 10 μm. The etched wafers were measured for flatness,surface roughness, pit depth, and waviness in order to evaluate theeffect of etching. The results are shown in Table 1.

The actual etching amounts of the alkali etching and the acid etchingwere as follows.

The etching amount of the alkali etching (target value: 20 μm): numberof samples: 51, average value: 20.1 μm, average value ±3σ: 18.1-22.1 μm.The etching amount of the acid etching (target value: 10 μm): number ofsamples: 107, average value: 9.8 μm, average value ±3σ: 8.3-11.3 μm.

Flatness (TTV, LTV) was measured by use of a flatness measuring device(U/G9500, U/S9600, products of ADE Corp.) Surface roughness (Ra) wasmeasured by use of a universal surface shape measuring device (Type:SE-3C, product of Kosaka Laboratory Co.).

Further, waviness was measured by use of the universal surface shapemeasuring device (Type: SE-3C, product of Kosaka Laboratory Co.).Specifically, a central area of the surface of a wafer (diameter: 200mm) was traced for 60 mm through use of a stylus within in order tomeasure the surface shape while the component of fine surface roughnesswas eliminated.

Waviness is defined as shown in FIG. 6. The vertical position of a startpoint and an end point of measurement that are determined to have thesame height is assumed as the origin in the vertical or heightdirection. The absolute values Y₁ to Y₂₉ of displacement from the originare measured at intervals of 2 mm. The average Y of the absolute valuesY₁ to Y₂₉ represents waviness.

TABLE 1 Items Pit TTV LTV_(max) ¹⁾ Ra depth Waviness Measured Ave. Ave.Ave. Max. Ave. Ex. No. wafers (μm) (μm) (μm) (μm) (μm) Example 1 50 0.990.55 0.18 5.5 0.062 Comparative 50 1.28 0.70 0.11 4.5 — example 1Comparative 50 0.93 0.38 0.24 8.2 — example 2 Example 2 50 0.56 0.270.22 5.8 0.033 Example 3 50 0.56 0.27 0.19 5.2 0.023 Note ¹⁾: Maximumvalue among values measured in cells of 20 × 20 mm over the entire wafersurface.

Comparative Example 1

Alkali etching was first performed under the same conditions as inExample 1 except that the target etching amount was set to 4 μm, andimmediately after the alkali etching, acid etching was performed with atarget etching amount being set to 36 μm. There was not performed atreatment for making the surface of the wafers hydrophilic through useof an aqueous solution of hydrogen peroxide. The results of measurementperformed for the thus-etched wafers are shown in Table 1.

Comparative Example 2

Only the alkali etching employed in Example 1 with a target etchingamount being set to 20 μm was performed for wafers. The results ofmeasurement performed for the thus-etched wafers are shown in Table 1.

Table 1 demonstrates the following. When only alkali etching isperformed (Comparative Example 2), although the flatness of each waferis good, the surface roughness deteriorates, and especially, the depthof locally formed deep pits increases. When acid etching is performedafter alkali etching (Comparative Example 1), the flatness of waferbecomes considerably worse because of the large acid etching amount. Bycontrast, when alkali etching and acid etching are performed with theiretching amounts being properly set (Example 1), well-balanced resultsare obtained in terms of flatness, surface roughness, and depth of deeppits. Further, observation of the surface shape through use of amicroscope reveals that the wafers obtained in Example 1 have a surfaceshape smoother than that of the wafers obtained in Comparative Example 2and as smooth as that of the wafers obtained in Comparative Example 1.

Example 2

The following etching treatment was performed for wafers having adiameter of 8 inches that had undergone lapping (#1200 lapping abrasivegrains).

First, the wafers were immersed in an NaOH aqueous solution(concentration: 50 wt. %) at 85° C. for 450 seconds in order to performalkali etching with a target etching amount being set to 20 μm.Subsequently, the wafers were dipped into an aqueous solution of 0.3%hydrogen peroxide in order to make the surface of the wafershydrophilic. Finally, the wafers were immersed into areaction-controlled acid etchant obtained through dissolution of 27.5g/l of silicon into a mixed acid (50% hydrofluoric acid: 70% nitricacid: 99% acetic acid=1:2:1 (volume ratio)) in order to performreaction-controlled acid etching with a target etching amount being setto 10 μm. The etched wafers were measured for flatness, surfaceroughness, pit depth, and waviness in order to evaluate the effect ofetching. The results are shown in Table 1. The results demonstrate thatthe reaction-controlled acid etching is especially effective forimprovement of flatness (TTV, LTV_(max)) and waviness.

Example 3

Alkali etching and reaction-controlled acid etching were performed undersame conditions as in Example 2 for 8-inch wafers that undergone lappingthrough combined use of #1200 abrasive grains and #1500 abrasive grains.The etched wafers were measured for flatness, surface roughness, pitdepth, and waviness in order to evaluate the effect of etching. Theresults are shown in Table 1. The results obtained in this exampledemonstrate that the reaction-controlled acid etching is especiallyeffective for improvement of flatness (TTV, LTV_(max)) and waviness.

Example 4

The following etching treatment was performed for wafers having adiameter of 8 inches that had undergone lapping (#1200 lapping abrasivegrains).

First, the wafers were immersed in a reaction-controlled acid etchantobtained through dissolution of 26 g/l of silicon into a mixed acid (50%hydrofluoric acid: 70% nitric acid: 99% acetic acid=1:2:1 (volumeratio)) at 35° C. for 150 seconds in order to performreaction-controlled acid etching with a target etching amount being setto 20 μm. Subsequently, the wafers were immersed into adiffusion-controlled acid etchant obtained through dissolution of 10 g/lof silicon into a mixed acid (50% hydrofluoric acid: 70% nitric acid:99% acetic acid=1:2:1 (volume ratio)) in order to performdiffusion-controlled acid etching with a target etching amount being setto 10 μm. The etched wafers were measured for flatness, surfaceroughness, pit depth, and waviness in order to evaluate the effect ofetching. The results are shown in Table 2.

The actual etching amounts of the reaction-controlled acid etching andthe diffusion-controlled acid etching were as follows.

Etching amount of the reaction-controlled acid etching (target value: 20μm): number of samples: 50, average value: 20.1 μm, average value ±3σ:18.1-22.1 μm. Etching amount of the diffusion-controlled acid etching(target value: 10 μm): number of samples: 50, average value: 9.8 μm,average value ±3σ: 8.3-11.3 μm.

Flatness (TTV, LTV) was measured by use of a flatness measuring device(U/G9500, U/S9600, products of ADE Corp.). Surface roughness (Ra) wasmeasured by use of a universal surface shape measuring device (Type:SE-3C, product of Kosaka Laboratory Co.).

TABLE 2 Items Pit TTV LTV_(max) ¹⁾ Ra depth Measured Ave. Ave. Ave. Max.Ex. No. wafers (μm) (μm) (μm) (μm) Example 4 50 1.02 0.56 0.20 6.4Comparative 50 1.35 0.75 0.15 5.2 example 3 Comparative 50 0.98 0.400.29 10.1 example 4 Note ¹⁾: Maximum value among values measured incells of 20 × 20 mm over the entire wafer surface.

Comparative Example 3

Reaction-controlled acid etching was first performed under the sameconditions as in Example 4 except that the target etching amount was setto 4 μm, and immediately after the reaction-controlled acid etching,diffusion-controlled acid etching was performed with a target etchingamount being set to 26 μm. The results of measurement performed for thethus-etched wafers are shown in Table 2.

Comparative Example 4

Wafers were subjected to only the reaction-controlled acid etchingemployed in Example 4 with a target etching amount being set to 30 μm.The results of measurement performed for the thus-etched wafers areshown in Table 2.

Table 2 demonstrates the following. When only reaction-controlled acidetching is performed (Comparative Example 4), although the flatness ofwafer is good, the surface roughness deteriorates, and especially, thedepth of locally formed deep pits increases. When diffusion-controlledacid etching is performed after reaction-controlled acid etching(Comparative Example 3), the etching amount of the diffusion-controlledacid etching is excessive, so that the flatness of wafer deterioratesconsiderably. By contrast, when reaction-controlled acid etching anddiffusion-controlled acid etching are performed with their etchingamounts being properly set (Example 4), well-balanced results areobtained in terms of flatness, surface roughness, and depth of deeppits. Further, observation of the surface shape through use of amicroscope reveals that the wafers obtained in Example 4 have a surfaceshape smoother than that of the wafers obtained in Comparative Example 4and as smooth as that of the wafers obtained in Comparative Example 3.

The present invention is not limited to the above-described embodiment.The above-described embodiment is a mere example, and those having thesubstantially same structure as that described in the appended claimsand providing the similar action and effects are included in the scopeof the present invention.

For example, additives such as surfactants may be added to the alkalietchants and the acid etchants used in the above-described embodiments.More specifically, when nitrite such as NaNO₂ is added to the alkalietchant, the depth of pits can be reduced more effectively. When afluorine-contained or nonionic surfactant is added to the acid etchant,generation of stain can be reduced more effectively.

In the above-described one embodiment, a mixed acid aqueous solutioncomposed of hydrofluoric acid, nitric acid, acetic acid, and water isdescribed as an example of the acid etchant. However, similar effectsare attained even when there is used a mixed acid aqueous solutioncomposed of hydrofluoric acid, nitric acid, and water but does notcontain acetic acid.

In the above-described the other embodiment, an etchant obtained throughdissolution of silicon into a mixed acid aqueous solution composed ofhydrofluoric acid, nitric acid, acetic acid, and water is described anexample of the etchants used in the reaction-controlled acid etching andthe diffusion-controlled acid etching. However, the present inventioncan be applied to the case where there is used an etchant obtainedthrough addition of acetic acid, phosphoric acid, or sulfuric acid intoa three-component mixed acid aqueous solution composed of hydrofluoricacid, nitric acid, and water.

Although the above-described embodiments are focused on semiconductorsilicon wafers, the present invention is not limited thereto and can beapplied to wafers of other semiconductor material such as a compoundsemiconductor (e.g., Ge, GaAs, Gap, InP)

What is claimed is:
 1. A method of processing a semiconductor wafersliced from a monocrystalline ingot, said method comprising at least thesteps of chamfering, lapping, etching, mirror-polishing, and cleaning,wherein in said etching step reaction-controlled acid etching is firstperformed and then diffusion-controlled acid etching is performed; andan etching amount of the reaction-controlled acid etching is greaterthan an etching amount of the diffusion-controlled acid etching.
 2. Amethod of processing a semiconductor wafer according to claim 1, whereinthe etching amount of the reaction-controlled acid etching is 10-30 μm,and the etching amount of the diffusion-controlled acid etching is 5-20μm.
 3. A method of processing a semiconductor wafer according to claim2, wherein in each of the reaction-controlled acid etching and thediffusion-controlled acid etching, there is used an etchant obtainedthrough addition of silicon into a mixed acid aqueous solution composedof hydrofluoric acid, nitric acid, acetic acid, and water, and thesilicon concentration of the etchant used in the reaction-controlledacid etching is higher than that of the etchant used in thediffusion-controlled acid etching.
 4. A method of processing asemiconductor wafer according to claim 3, wherein the siliconconcentration of the etchant used in the reaction-controlled acidetching is 20-30 g/l, and the silicon concentration of the etchant usedin the diffusion-controlled acid etching is 5-15 g/l.
 5. A method ofprocessing a semiconductor wafer according to claim 1, wherein in eachof the reaction-controlled acid etching and the diffusion-controlledacid etching, there is used an etchant obtained through addition ofsilicon into a mixed acid aqueous solution composed of hydrofluoricacid, nitric acid, acetic acid, and water, and the silicon concentrationof the etchant used in the reaction-controlled acid etching is higherthan that of the etchant used in the diffusion-controlled acid etching.6. A method of processing a semiconductor wafer according to claim 5,wherein the silicon concentration of the etchant used in thereaction-controlled acid etching is 20-30 g/l, and the siliconconcentration of the etchant used in the diffusion-controlled acidetching is 5-15 g/l.